The present invention relates to a Bluetooth system; and more particularly, to an apparatus for compensating a channel distortion for use in a Bluetooth system.
Recently, with the rapid advent of information age accompanied with fast development of various communication technologies, industries have taken strong interests in wireless personal area networks (WPAN) such as the so-called Bluetooth and shared wireless access protocol (SWAP). In particular, the Bluetooth system is focused on a low cost, simple hardware and robustness facilitating protected ad-hoc connections for stationary and mobile communication environments.
The Bluetooth system has three main application areas: a wire replacement, a local area network (LAN) access point and a personal area network. In the so-called Bluetooth ad-hoc network or piconet, data is conveyed in a packet having access codes, a header and a payload. There are two link types: a synchronous connection-oriented (SCO) link and an asynchronous connection-less (ACL) link. The SCO link is a point-to-point link between a master and a single slave. The ACL link is a point-to-multipoint link between a master and slaves in a piconet.
The Bluetooth system usually adopts a slotted time division duplex (TDD) scheme for full duplex transmission, wherein in this case, a length of each slot is 625 xcexcs and two slots form one frame. A fast frequency hopping scheme of 1600 hops/s and a short data packet format are used for robustness in a noisy and interference environment. Also, a forward error correction (FEC) and an automatic repeat request (ARQ) are used as error correction schemes. The purpose of a FEC scheme on a data payload is to reduce the number of retransmission for improving throughput thereof. The data transmitted has a gross bit rate of 1 Mbit/sec.
A Gaussian-shaped frequency shift keying (GFSK) modulation is applied to minimize a transceiver complexity thereof. The nominal and supported range of the Bluetooth system is from 10 cm to 10 m; but the range can be extended to 100 m with an external power amplifier. A binary GFSK with modulation index between 0.28 and 0.35 is employed for a simple and small transceiver implementation.
FIG. 1 shows a block diagram of a GFSK transmitter for use in a conventional Bluetooth system. The GFSK transmitter includes a Bluetooth link controller 11, a packet generator 13, a Gaussian low pass filter (LPF) 15, an integrator 17 and a modulator 19. The packet generator 13 generates a packet signal under the control of the Bluetooth link controller 11. The Gaussian LPF 15 filters the packet signal to thereby provide a filtered signal g(t). Then, the integrator 17 performs integration on the filtered signal g(t) to thereby feed an integrated signal. The modulator 19 modulates the integrated signal to thereby generate a GFSK signal p(k). A binary. GFSK with modulation index between 0.23 and 0.35 is employed for a simple and small transceiver implementation.
In the above, the p(t) can be written as:       p    ⁡          (      t      )        =      Re    ⁢          {                                                  2              ⁢              E                        T                          ⁢                  ⅇ                      j2π            ⁢                          {                                                                    f                    c                                    ⁢                  t                                +                                  h                  ⁢                                                            ∫                                              -                        ∝                                            t                                        ⁢                                                                  g                        ⁡                                                  (                          τ                          )                                                                    ⁢                                              xe2x80x83                                            ⁢                                              ⅆ                        τ                                                                                                        }                                          }      
wherein E is an energy per a symbol; T is a symbol period; fc is a carrier frequency; h is a modulation index; and g(t) is the output of the Gaussian LPF 15. The g(t) can be expressed as:       g    ⁡          (      t      )        =            ∑              k        =                  -          ∝                    ∞        ⁢          xe2x80x83        ⁢                  a        k            ⁢              v        ⁡                  (                      t            -            kT                    )                    
wherein ak=xc2x11; and             v      ⁡              (        t        )              =                  1        2            ⁢              {                              erf            ⁡                          (                                                -                  λ                                ⁢                                  xe2x80x83                                ⁢                                  B                  b                                ⁢                T                            )                                +                      erf            ⁡                          (                              λ                ⁢                                  xe2x80x83                                ⁢                                                      B                    b                                    ⁡                                      (                                          t                      +                      T                                        )                                                              )                                      }              ,      λ    =                  2                  ln          ⁢                      xe2x80x83                    ⁢          2          ⁢          π                      ,
BbT=0.5, Bb being a 3 dB bandwidth of GLPF and       erf    ⁡          (      t      )        =            ∫      0      t        ⁢                  2                  π                    ⁢              xe2x80x83            ⁢              ⅇ                  -                      t            2                              ⁢                        ⅆ          t                .            
Meanwhile, in the conventional Bluetooth system employed in an indoor environment such as home, office or airport, it is assumed that a statistical channel modeling for the Bluetooth system is performed as a multi-path channel model; and received signals form groups of clusters. A low-pass equivalent channel impulse response can be given       c    ⁡          (      t      )        =            ∑              l        =        0            ∞        ⁢          xe2x80x83        ⁢                  ∑                  k          =          0                ∞            ⁢              xe2x80x83            ⁢                        γ          kl                ⁢                  ⅇ                      j            ⁢                          xe2x80x83                        ⁢                          θ              kl                                      ⁢                  δ          ⁡                      (                          t              -                              T                l                            -                              τ                kl                                      )                              
wherein T1 is an arrival time of an l-th cluster; xcfx84kl is an arrival time of a k-th ray measured from the beginning of the l-st cluster; xcex8kl is a phage shift; and xcex3kl is a power gain of the k-th ray in the l-st cluster. It is assumed that the Bluetooth system operates in the indoor environment with an rms delay spread of 50 ns, a maximum delay spread of 300 ns and Doppler spread of 10 Hz. The modulated GFSK signal is transmitted at a 1 Mbit/s rate in 625 xcexcs slot size, which makes the channel to be fixed within a slot.
FIG. 2 depicts a conventional channel modeling of the Bluetooth system. A received GFSK signal s(t) which has been changed while passing the transmission channel, as shown in FIG. 2, can be given by:                               s          ⁡                      (            t            )                          =                                                            m                ⁡                                  (                  t                  )                                            ⁢                              c                ⁡                                  (                  t                  )                                                      +                          n              ⁢                              (                t                )                                              =                      xe2x80x83                    ⁢                                                                                          2                    ⁢                    E                                    T                                            ⁢                              C                ⁡                                  (                  t                  )                                            ⁢                              ⅇ                                  j                  ⁡                                      (                                                                  φ                        ⁡                                                  (                                                      t                            ,                                                          α                              _                                                                                )                                                                    +                                                                        φ                          c                                                ⁡                                                  (                          t                          )                                                                                      )                                                                        +                                          N                ⁡                                  (                  t                  )                                            ⁢                              ⅇ                                  j                  ⁢                                      xe2x80x83                                    ⁢                                                            φ                      n                                        ⁡                                          (                      t                      )                                                                                                                                  =                  xe2x80x83                ⁢                                                                              A                  2                                ⁡                                  (                  t                  )                                            +                                                B                  2                                ⁡                                  (                  t                  )                                                              ⁢                      ⅇ                          j              ⁢                              xe2x80x83                            ⁢                              tan                                  -                  1                                            ⁢                                                B                  ⁡                                      (                    t                    )                                                                    A                  ⁡                                      (                    t                    )                                                                                          
wherein
c(t)=C(t)ejxcfx86c(t) is a component of the channel distortion;
n(t)=N(t)ejxcfx86m(t) is an additive white Gaussian noise (AWGN);
xcfx86(t,{overscore (xcex1)})=2xcfx80h∫∞g(xcfx84)dxcfx84;       m    ⁡          (      t      )        =                              2          ⁢          E                T              ⁢          ⅇ              j        ⁢                  xe2x80x83                ⁢                  φ          ⁡                      (                          t              ,                              α                _                                      )                              
xe2x80x83is an equivalent complex envelope of p(t); and             A      ⁡              (        t        )              =          {                                                                  2                ⁢                E                            T                                ⁢                      C            ⁡                          (              t              )                                ⁢                      cos            ⁡                          (                                                φ                  ⁡                                      (                                          t                      ,                                              α                        _                                                              )                                                  +                                  φ                  ⁡                                      (                    t                    )                                                              )                                      +                              N            ⁡                          (              t              )                                ⁢                      cos            ⁡                          (                                                φ                  n                                ⁡                                  (                  t                  )                                            )                                          }        ,      B    ⁡          (      t      )        =            {                                                                  2                ⁢                E                            T                                ⁢                      C            ⁡                          (              t              )                                ⁢                      sin            ⁡                          (                                                φ                  ⁡                                      (                                          t                      ,                                              α                        _                                                              )                                                  +                                  φ                  ⁡                                      (                    t                    )                                                              )                                      +                              N            ⁡                          (              t              )                                ⁢                      sin            ⁡                          (                                                φ                  n                                ⁡                                  (                  t                  )                                            )                                          }        .  
A GFSK modulation system usually uses an FM discriminator. FIG. 3 represents a structure of a conventional GFSK modulator 300. The GFSK modulator 300 includes a hard limiter 310, an FM discriminator 320 and an LPF 330. The hard discriminator 310 compensates an amplitude of the received signal s(t). The FM discriminator 320 detects a phase of the compensated s(t) to thereby extract desired information. The following terms Sc (t) and xcfx86(t) are related to the desired information, which can be expressed by the following equations:                                                         S              c                        ⁡                          (              t              )                                =                      xe2x80x83                    ⁢                                                    s                ⁡                                  (                  t                  )                                                            "LeftBracketingBar"                                  s                  ⁡                                      (                    t                    )                                                  "RightBracketingBar"                                      ⁢                          ⅇ                              j                ⁢                                  xe2x80x83                                ⁢                                  tan                                      -                    1                                                  ⁢                                                      B                    ⁡                                          (                      t                      )                                                                            A                    ⁡                                          (                      t                      )                                                                                                          ,                                          ϕ          ⁡                      (            t            )                          =                  xe2x80x83                ⁢                              ⅆ                          {                                                tan                                      -                    1                                                  ⁢                                                      B                    ⁡                                          (                      t                      )                                                                            A                    ⁡                                          (                      t                      )                                                                                  }                                            ⅆ            t                                                  =                  xe2x80x83                ⁢                              1                          1              +                                                (                                                            B                      ⁡                                              (                        t                        )                                                                                    A                      ⁡                                              (                        t                        )                                                                              )                                2                                              ⁢                                                                                          B                    xe2x80x2                                    ⁢                                      (                    t                    )                                    ⁢                                      A                    ⁡                                          (                      t                      )                                                                      -                                                      B                    ⁡                                          (                      t                      )                                                        ⁢                                                            A                      xe2x80x2                                        ⁡                                          (                      t                      )                                                                                                                    A                  2                                ⁡                                  (                  t                  )                                                      .                              
Referring to the above equations for the Sc (t) and xcfx86(t), it can be understood that a demodulated GFSK signal may be distorted by the channel. In other words, the distortion of the phase and amplitude of a signal transmitted through a channel deteriorates the performance of the demodulator. Further, in the conventional Bluetooth system employed in an indoor environment such as home, office or airport, the distortion of the phase and amplitude of a transmitted signal through a channel can be severely deteriorated due to the reflection, refraction, diffraction or dispersion therein. Accordingly, it is necessary to prepare a channel distortion compensation apparatus in the Bluetooth system.
In a conventional apparatus for compensating the channel distortion in the Bluetooth system, the information for the channel is needed. A pilot signal or a training signal is employed to offer the information for the channel. But, in this case, a structure of a transmitter affects the information for the channel to thereby entail the complexity of the receiving end. In the case that the pilot signal is used, the channel information can be obtained by estimating the pilot signal passed through the channel, wherein a signal having a frequency are transmitted from the sending end to the receiving end.
To use a channel compensation scheme employing a pilot signal or a training signal, a structure of a receiver should be constructed to provide information therefor. But, the structure of the receiver is limited since the structure is generally predetermined with a preset specification. Further, when a pilot signal is used, since a pilot signal should be transmitted with a preset frequency, the efficiency thereof is deteriorated. When a training signal is used, since a training signal should be included in a packet signal to be transmitted, the efficiency thereof is deteriorated. In particular, in a commercial Bluetooth system, since a pilot signal or a training signal is not used due to a power consumption problem, synchronization detection is impossible.
It is, therefore, an object of the present invention to provide an apparatus for compensating a channel distortion by employing access codes for use in a Bluetooth system.
In accordance with the present invention, there is provided an apparatus for compensating a channel distortion in a Bluetooth system to process a received signal having access codes, comprising:
means for multiplying the received signal by a previously obtained channel distortion compensation signal to thereby provide a multiplied signal;
means for demodulating access codes of the multiplied signal to thereby output demodulated received access codes as a demodulated signal;
means for detecting correlation values between the demodulated received access codes and the access codes of the received signal previously stored therein to detect a start point of the received signal, thereby providing detected access codes having corresponding correlation values greater than a predetermined threshold and providing an enable signal if there are one or more correlation values greater than the predetermined threshold; and
means, in response to the enable signal, for performing a channel distortion compensation based on the detected access codes and a reference signal obtained by re-modulating the access codes of the received signal to thereby provide a channel distortion compensation signal.